he regenerative receiver offered highest sensitivity per circuit element, though, and experiments aimed at increasing its sensitivity resulted in the super regenerative circuit This circuit actually oscillates, but its oscillation is interrupted or "quenched" at a supersonic frequency. As a result, you are able to hear the actual electron noise in the antenna wire. Any more sensitivity than this would be unusable anyway.
Unfortunately, both selectivity and stability on a superregen are poor. In addition, this receiver simultaneously performs as a low-powered transmitter, interfering with all other receivers in the area tuned to the same frequency.
The superhet is a combination of the *RF? the regenerative circuit, and the crystal set. The block diagram of Fig. 1 may be helpful here. The front end consists of any rf stages, plus a mixer which is similar to a regenerative receiver except that it is adjusted to produce an "intermediate frequency" output instead of an audible beat note. Some front ends use only a single tube or transistor as the mixer and call it a "converter, while other designs use separate oscillator and mixer elements. In these, the oscillator is called the "local oscillator^ or "fif oscillator", and the mixer is known as the mixer.
The if strip is almost identical to a TRF receiver, except that it is permanently tuned to a single frequency and does not include a detector. The if frequency is usually relatively low; many circuits use a standard if of 455 kHz, but higher frequencies in the 1 to 10 MHz range are becoming more popular. In general, the lower the frequency the greater the gain and selectivity, but we'll go into this in more detail later.
The detector, for AM reception, is almost always a direct equivalent of the crystal set. For CW and SSB, product detectors (which are special types of mixers) are used.
The superhet combines the best features of all the types of receivers it combines, while escaping for the most part their problems. It offers excellent sensitivity, and if properly designed can have extreme selec-
tivity and stability as well. For this reason, it has displaced all other designs as the "standard" receiver circuit—unless factors such as simplicity or economy happen to outweigh performance requirements in some special application such as the Sixer or Twoer!
How Docs The Front End Operate? The "front end" of a superhet is the key to the entire receiver, and its operation contributes most noticeably to the operation of the receiver as a whole. Fig. 2 is a block diagram of a typical "front end"; not all superhets include separate tubes for each block of Fig, 2. but all the functions are performed,
The basic idea behind the superhet is that of "beating" or mixing two radio frequencies in order to produce a sum or difference frequency, which carries all the modi i hit ion of both original frequencies. If one of the two original frequencies is the desired signal, and if the other is unmodulated and at a fixed distance in the spectrum from ¡he first, tlien the difference frequency will carry the modulation of the desired signal, but will always be at a fixed frequency,
This fixed or "intermediate" frequency is equal to the separation between the frequency of the desired signal and that of the unmodulated second signal.
The functions shown in Fig. 2 break down as follows: The rf Stages amplify the desired signal, thus increasing receiver sensitivity, and provide some selectivity as well The Local Oscillator provides the unmodulated second signal; this oscillator is always tuned to a frequency which is separated from that oi the desired signal by the if frequency, but may at times be tuned to a higher frequency than the signal (high-side injection) and at other times to a lower frequency than the signal (low-side injection). The Mixer combines the two rf signals to produce both the sum and difference frequencies; a tuned circuit in the Mixer's output portion selects only the difference frequency for passage to the if strip,
Since the local oscillator may operate on either the high side or the low side of the incoming signal, it follows that a single setting of the oscillator frequency permits reception of incoming signals at two different frequencies. One of these is above the oscillator frequency bv the amount of the if: for it, the oscillator provides low-side injection. The other is below the oscillator frequency by the amount of the if, where the oscillator provides high-side injection.
One of these two frequencies represents the desired signal. The other represents an undesired response called the "image", and is one of the major disadvantages of the superhet receiver. The image is always separated from the desired signal by twice the intermediate frequency: If the if is low to provide gain and selectivity easily, the image will be close to the desired signal. If the if is made higher to move the image away from the desired response (the more separated the two signals are, the more readily the image can be rejected by the tuned circuits in the rf stages) , both gain and selectivity suffer,
A standard if frequency is 455 kHz. In a receiver using this ify the local oscillator is always tuned 455 kHz away from the frequency to be received, and the images are always 910 kHz away from the desired responses,
Most BCB receivers operate with high-side injection to overcome tracking problems; this means that image response can be observed easily if you have a high-powered BC station operating above 1410 kH near you. Simply tune to the low end of the band. At 500 kHz on the dial a strong 1410-kHz signal will come through. At 690 kHz on the dial, powerful 1600-kHz signals can be heard. This is normal for a superhet.
To receive an incoming signal at 7.2 MHz with a 455-kHz if, you could set the local oscillator to either 6.745 MHz for low-side injection (with image at 6.290 MHz) or 7.655 M z for high-side injection (with image at 8.110 MHz),
Both image frequencies are fairly close to the desired frequency in the previous example. By the time you move the operating frequency up to 10 meters or above, the image response may be just as strong as that to the desired signal. For this reason, many receivers use a higher //. If a high if is used for image control, and then converted to a lower if for its advantages, the receiver is known as a "double conversion" receiver, incidentally, any receiver used with an outboard converter (as for YHF operation) is double conversion, since the outboard con-verier effectively becomes the receiver's front end.
If the if is properly chosen, the image situation can be used to advantage. For instance, to receive signals on either 40 or 80 meters, an if of 1750 kHz can be chosen. This puts the images 3.5 MHz away from the desired response, and if the oscillator frequency is selected to give high-side injection on 80 and low-side on 40, then one response will cover 3.5 to 4.0 MHz while the other response gets 7.0 to 7.5 MHz. No bandswitch is necessarvl
Similar reasoning led to the choice of 9 MHz as the "standard" frequency for SSB sideband generation. Use of a 5-MHz conversion oscillator permitted output on either 80 or 20 meters without a bandswitchT by using wimage" response techniques.
The local oscillator is important to the superhet for a number of reasons, Its frequency stability, for example, determines the stability of the entire receiver, because il the local oscillator drifts the // produced by a stable signal will drift out of the ¿/-strip passband. It must also be relatively low in noise modulation, since any modulation present on it will appear in the output together with that of the desired signal.
The mixer must convert frequencies properly, but in the HF range its characteristics are not particularly important. At YHF, mixers must have low noise if full receiver sensitivity is to be used. At any frequency, they must be overload-resistant to avoid interference, but most common circuits resist overload to an acceptable extent.
The rf stages may be omitted in inexpensive receivers; in this case their key function is taken over by the mixers tuned input circuit. This function is to select the desired frequency and reject as much as possible of the image response.
If rf stages are present, they add selectivity so far as images and interference by cross-modulation and mixer overload are concerned, but they have little effect upon adjacent-signal selectivity . That occurs 111 the if strip.
One major effect of the rf stages is to determine receiver sensitivity. Sensitivity is not the same as gain; sensitivity measures the weakest signal which can be received while gain measures the amount of amplification available through the receiver. A high-gain receiver without an rf stage and with a noisy mixer can easily fill the room with amplified noise, but it won't get many weak signals, A unit having lower gain, but with adequate if amplification to overcome any mixer noise, may appear to be dead—
until vou tune across a "down in the mud'
signal and copy it clearly.
Because of the difference between sensitivity and gain, simply counting the number of rf stages isn't much of a guide. A single, good rf stage is better than three which contribute little but noise.
If a receiver will show an increase in noise when an antenna is connected to it, its sensitivity is probably already at the usable limit. If no change can be detected, though, addition of another rf stage may prove profitable in increasing (and thus improving) sensitivity, Phis may be done by adding a preselector. outboard. A "preselector" is nothing but an outboard rf stage (or stages),
Xote that sensitivity can be affected only by the rf stages; gain, on the other hand, can be increased in the if strip, or even after the audio is recovered from the signal. If no rf stage is present, it may be possible to modify the mixers operation to improve sensitivity—but addition of a preselector is a far better solution.
What Does The if Strip Do? The front end selects any desired signal frequency within its operating range and converts it to a single fixed intermediate frequency. The if strip, then, accepts this intermediate frequency signal and amplifies it to a level suitable for the detector circuits. At the same time, unwanted signals at adjacent frequencies are rejected.
The // strip thus serves two purposes of equal importance, it provides gain for the selected signal—not to be confused with sensitivity—and also provides the major part of the receiver's selectivity.
Fig, 3 shows the arrangement of a typical 2-stage if strip. Any specific receiver may have more or less stages of if than this; the minimum is a single transformer, which provides only selectivity and no gain, while the maximum normally encountered is three.
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